Project Summary Repeated, uncontrollable stress is a key contributor to depression and anxiety disorders, including posttraumatic stress disorder. Stress causes a series of psychological and physiological adaptations that can be chronic and sustained, and likely contribute to the vulnerability to subsequent bouts of depression and anxiety. On the other hand, some individuals can be exposed to stress repeatedly and appear to be relatively resilient. Given that the serotonin system plays a key role in stress responses and the pharmacological treatment of stress disorders, we presume that the adaptations associated with vulnerability and resilience are represented in cellular and molecular plasticity in the raphe neurons of individuals following exposure to stress, especially in pathways that are involved in signal transduction. This project will use mouse behavioral and molecular strategies to investigate these adaptations and how resilience might be promoted while vulnerability might be prevented or reversed. Aim 1 will examine how modulating signal transduction pathways within serotonergic neurons might alter the behavioral phenotype of animals exposed to repeated social defeat stress. We proposed to express DREADDs that activate Gi or Gs signaling pathways selectively in serotonergic neurons (ePet1-Cre mice). The effects of DREADD activation prior to repeated social defeat on stress responsiveness in behavioral assays of depression-like and anxiety-like behaviors. This Aim will provide new mechanistic and therapeutic clues regarding how plasticity in the serotonin system confers vulnerability or resilience to stress. In Aim 2 we will evaluate whether DREADD-mediated inhibition of serotonergic neurons in the interval after repeated social defeat stress can reverse the enhanced vulnerability to subsequent stressors, using corticosterone responses to restraint and immobility in the forced swim test to assess different domains of stress reactivity. Aim 3 will selectively interrogate the translation of mRNA by selectively expressing ?RiboTag? in the serotonergic neurons of ePET1-Cre mice. RiboTag is an epitope-tagged ribosomal protein that allows for antibody-mediated pull down only of polysomes from target cells derived from a complex cell homogenate (in this case a fresh midbrain tissue punch). Enrichment of mRNA from serotonergic neurons will allow us to perform RNA-seq to investigate the totality of actively translated mRNA at key points during and after stress exposure. We will focus on mRNAs associated with excitability, signal transduction pathways, and cytokine responses, and use pathway analysis to identify co-regulation of mRNA translation in response to stress; this is intended to provide new mechanistic information and new targets for therapeutic development.